1. Field of the Invention
The present invention relates to telecommunication receivers. More particularly, the present invention relates to analog-to-digital converters in demodulators.
2. Background Art
In telecommunications, modulation is the process of varying a periodic waveform, i.e., a tone, in order to use that signal to convey a message, in a similar fashion as a musician may modulate the tone from a musical instrument by varying its volume, timing and pitch. Typically, a high-frequency sinusoid waveform is used as carrier signal. The three key parameters of a sinusoidal or sine wave are its amplitude (“volume”), phase (“timing”) and its frequency (“pitch”), all of which can be modified in accordance with a low frequency information signal to obtain the modulated signal.
A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator. In digital modulation, an analog carrier signal is modulated by a digital bit stream. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of alternative symbols. The most fundamental digital modulation techniques are: phase-shift keying (PSK), frequency-shift keying (FSK), amplitude-shift keying (ASK), and Quadrature amplitude modulation (QAM).
Among these digital modulation schemes, PSK is a common digital modulation that conveys data by changing, or modulating, the phase of a reference signal or the carrier wave. PSK uses a finite number of phases, each assigned a unique pattern of binary bits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal. Alternatively, instead of using the bit patterns to set the phase of the wave, it can instead be used to change it by a specified amount. The demodulator then determines the changes in the phase of the received signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying (DPSK).
A convenient way to represent PSK schemes is on a constellation diagram. This shows the points in the Argand plane where, in this context, the real and imaginary axes are termed the in-phase (I) and quadrature-phase (Q) axes, respectively, due to their 90° separation. Such a representation on perpendicular axes lends itself to straightforward implementation. The amplitude of each point along the in-phase axis is used to modulate a cosine (or sine) wave and the amplitude along the quadrature-phase axis is used to modulate a sine (or cosine) wave. In PSK, the constellation points chosen are usually positioned with uniform angular spacing around a circle. This gives maximum phase-separation between adjacent points and thus the best immunity to corruption. The constellation points are positioned on a circle so that they can all be transmitted with the same energy. In this way, the moduli of the complex numbers they represent will be the same and thus so will the amplitudes needed for the cosine and sine waves. Two common examples are “binary phase-shift keying” (BPSK) which uses two phases, and “quadrature phase-shift keying” (QPSK), which uses four phases, although any number of phases may be used.
Now, FIG. 1 illustrates conventional QPSK receiver 100, which includes tuner 150 and demodulator 160. As shown in FIG. 1, tuner 150 receives analog signal 104, which is a composite signal with magnitude and phase (or I and Q) information. Tuner 150 includes oscillator 108 for generating a local oscillator signal at the carrier frequency, which is provided to phase modifier 110 for generating the carrier frequency at 90° and 0° phase shifts. Decomposer 106 of tuner 150 utilizes the carrier frequency at the 0° phase shift to extract the in-phase or I component of composite signal 104 that can be further processed by amplifiers and filters 112 to provide I signal 116 to demodulator 160. Also, decomposer 107 of tuner 150 utilizes the carrier frequency at the 90° phase shift to extract the quadrature or Q component of composite signal 104 that can be further processed by amplifiers and filters 114 to provide Q signal 118 to demodulator 160.
As further shown in FIG. 1, demodulator 160 includes first analog-to-digital converter (ADC) 120 and second ADC 122, which receive I signal 116 and Q signal 118, respectively, from tuner 150. First ADC 120 converts I signal 116 to I bitstream 124 and, similarly, second ADC 122 converts Q signal 118 to Q bitstream 126. Demodulator 160 also includes processor 128, which receives I bitstream 124 and Q bitstream 126 for data processing.
The conventional implementation shown by FIG. 1 has several drawbacks. One significant disadvantage of QPSK receiver 100 is that it requires two ADCs, first ADC 120 for I signal 116, and second ADC 122 for Q signal 118. Due to variabilities in the manufacturing process utilized to produce first ADC 120 and second ADC 122, they cannot be expected to be exactly the same, nor to perform identically. Consequently, operational parameters of the individual ADCs typically will be different. For example, the offset voltages VosI and VosQ, and the converter gains AvI and AvQ of respective first and second ADCs 120 and 122, shown in FIG. 1, may differ. Resultant mismatches between the performance of first ADC 120 and second ADC 122 can affect the performance of demodulator 160, and in some cases may prevent demodulator 160 from performing to its operational specifications. That, in turn, may lead to lower manufacturing yield, or require further expenditures of resources flowing from the need to include additional circuitry to compensate for the variabilities.
Additional significant drawbacks to the conventional implementation shown in FIG. 1 is that ADC circuits are among the largest circuits in terms of circuit area, and among the most voracious in terms of power consumption. This is particularly true for ADC circuits used in high-speed applications. While these latter disadvantages, by themselves, are sufficient to make circuits requiring multiple ADCs quite undesirable, when taken together with the drawbacks resulting from manufacturing variability discussed previously, they make the conventional implementation shown by FIG. 1 impractical for high-speed applications.
One conventional approach to avoiding use of two ADC circuits to perform demodulation in a QPSK receiver is shown in FIG. 2. Demodulator 260, in FIG. 2 may be seen to correspond to demodulator 160, in FIG. 1. FIG. 2 shows demodulator 260 receiving I signal 216 and Q signal 218 from a tuner, not shown in FIG. 2. For the sake of simplicity, analogues to tuner 150 and its constituent elements shown in FIG. 1 are not reprised in FIG. 2. Demodulator 260 includes first sample-and-hold circuit 230 and second sample and hold circuit 232, which receive I signal 216 and Q signal 218, respectively. Demodulator 260 also includes single ADC 220 and processor 228, corresponding respectively to one of ADCs 120 or 122, and processor 128, of demodulator 160 in FIG. 1.
First and second sample-and-hold circuits 230 and 232 allow I signal 216 and Q signal 218 to have their respective values captured and delivered sequentially to ADC 220. ADC 220, which is shown to be operating at twice the frequency of first and second sample-and-hold circuits 230 and 232, then converts I signal 216 to corresponding bitstream data, and Q signal 218 to corresponding bitstream data. Processor 228 receives I and Q bitstream 224 for data processing.
The apparent advantage of the conventional solution to eliminating one ADC from a demodulator used in a QPSK receiver, shown in FIG. 2, is the absence of a second ADC. This apparent advantage is significantly qualified, however, by drawbacks introduced by the requirement that two sample-and-hold circuits be provided to mediate delivery of I signal 216 and Q signal 218 to ADC 220. Those drawbacks include offsets to the circuit area and power consumption savings achieved through elimination of one ADC, by the additional circuit area and power consumption required for implementation of the two new sample-and-hold circuits. Furthermore, while signal mismatch due to performance variations between two different ADCs is now eliminated, signal mismatch due to performance variations between two separate sample-and-hold circuits is introduced. Moreover, due to resolution losses associated with operation of ADC 220 at high frequencies in the conventional art, the requirement in demodulator 260 that ADC 220 operate at twice the frequency of first and second sample-and-hold circuits 230 and 232, limits that implementation to low to moderate frequency applications.
Accordingly, there is a need to overcome the drawbacks and deficiencies in the art by providing a demodulator for high speed applications.